Organic photovoltaic array and method of manufacture

ABSTRACT

The fabrication and characterization of large scale inverted organic solar array fabricated using all-spray process is disclosed, consisting of four layers; ITO-Cs 2 CO 3 -(P3HT:PCBM)-modified PEDPT:PSS, on a substrate. With PEDPT:PSS as the anode, the encapsulated solar array shows more than 30% transmission in the visible to near IR range. Optimization by thermal annealing was performed based on single-cell or multiple-cell arrays. Solar illumination has been demonstrated to improve solar array efficiency up to 250% with device efficiency of 1.80% under AM1.5 irradiance. The performance enhancement under illumination occurs only with sprayed devices, indicating device enhancement under sunlight, which is beneficial for solar energy applications. The semi-transparent property of the solar module allows for applications on windows and windshields, indoor applications, and soft fabric substances such as tents, military back-packs or combat uniforms, providing a highly portable renewable power supply for deployed military forces.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of prior-filed U.S. application Ser.No. 13/907,416, entitled “Organic Photovoltaic Array and Method ofManufacture”, filed on May 31, 2013, which is a continuation ofprior-filed International Application, Serial Number PCT/US2012/025028,entitled “Organic Photovoltaic Array and Method of Manufacture”, filedFeb. 14, 2012, which claims priority to U.S. Provisional PatentApplication 61/442,561, entitled, “Organic Photovoltaic Array and Methodof Manufacture”, filed 14 Feb. 2011, the contents of which are hereinincorporated by reference.

FIELD OF INVENTION

This invention relates to spray-manufactured organic solar photovoltaiccell. Specifically, the invention provides a novel method ofmanufacturing organic solar photovoltaic cells using spray-depositionand the organic solar photovoltaic cell resulting therefrom.

BACKGROUND OF THE INVENTION

In recent years, energy consumption has drastically increased, due inpart to increased industrial development throughout the world. Theincreased energy consumption has strained natural resources, such asfossil fuels, as well as global capacity to handle the byproducts ofconsuming these resources. Moreover, future demands for energy areexpected in greatly increase, as populations increase and developingnations demand more energy. These factors necessitate the development ofnew and clean energy sources that are economical, efficient, and haveminimal impact on the global environment.

Photovoltaic cells have been used since the 1970s as an alternative totraditional energy sources. Because photovoltaic cells use existingenergy from sunlight, the environmental impact from photovoltaic energygeneration is significantly less than traditional energy generation.Most of the commercialized photovoltaic cells are inorganic solar cellsusing single crystal silicon, polycrystal silicon or amorphous silicon.Traditionally, solar modules made from silicon are installed on rooftopsof buildings. However, these inorganic silicon-based photovoltaic cellsare produced in complicated processes and at high costs, limiting theuse of photovoltaic cells. These silicon wafer-based cells are brittle,opaque substances that limit their use, such as on window technologywhere transparency is a key issue. Further, installation is an issuesince these solar modules are heavy and brittle. In addition,installation locations, such as rooftops, are limited compared to thewindow area in normal buildings, and even less in skyscrapers. To solvesuch drawbacks, photovoltaics cell using organic materials have beenactively researched.

The photovoltaic process in OPV first starts from the absorption oflight mainly by the polymer, followed by the formation of excitons. Theexciton then migrates to and dissociates at the interface of donor(polymer)/acceptor (fullerene). Separated electrons and holes travel toopposite electrodes via hopping, and are collected at the electrodes,resulting in an open circuit voltage (Voc). Upon connection ofelectrodes, a photocurrent (short circuit current, Isc) is created.

Organic photovoltaic cells based on π-conjugated polymers have beenintensively studied following the discovery of fast charge transferbetween polymer and carbon C₆₀ (Sariciftci, et al., Science 1992, 258,1474; Yu, et al., Science 1995, 270, 1789; Yang and Heeger, Synth. Met.1996, 83, 85; Shaheen, et al., Appl. Phys. Lett. 2001, 78, 841;Padinger, et al., Adv. Funct. Mater. 2003, 13, 85; Brabec, et al., Appl.Phys. Lett. 2002, 80, 1288; Ma, et al., Adv. Funct. Mater. 2005, 15,1617; Reyes-Reyes, et al., High-efficiency photovoltaic devices based onannealed poly(3-hexylthiophene) and1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C₆₁ blends. Appl. Phys. Lett.2005, 87, 083506-9; Chen, et al., Polymer solar cells with enhancedopen-circuit voltage and efficiency. Nat. Photonics, 2009, 3(11),649-53). Conventional organic photovoltaic devices use transparentsubstrates, such as an indium oxide like indium tin oxide (ITO) or IZO,as an anode and aluminum or other metal as a cathode. A photoactivematerial including an electron donor material and an electron acceptormaterial is sandwiched between the anode and the cathode. The donormaterial in conventional devices is poly-3-hexylthiophene (P3HT), whichis a conjugated polymer. The conventional acceptor material is(6,6)-phenyl C₆₁ butyric acid methylester (PCBM), which is a fullerenederivative. Both the ITO and aluminum contacts use sputtering andthermal vapor deposition, both of which are expensive, high vacuum,technologies. In these photovoltaic cells, light is typically incidenton a side of the substrate requiring a transparent substrate and atransparent electrode. However, this limits the materials that may beselected for the substrate and electrode. Further, a minimum thicknessof 30 to 500 nm is needed to increasing conductivity. Moreover, theorganic photoelectric conversion layer is sensitive to oxygen andmoisture, which reduce the power conversion efficiency and the lifecycle of the organic solar cell. Development of organic photovoltaiccells, has achieved a conversion efficiency of 3.6% (P. Peumans and S.R. Forrest, Appl. Phys. Lett. 79, 126 (2001)).

The photovoltaic process in OPV first starts from the absorption oflight mainly by the polymer, followed by the formation of excitons. Theexciton then migrates to and dissociates at the interface of donor(polymer)/acceptor (fullerene). Separated electrons and holes travel toopposite electrodes via hopping, and are collected at the electrodes,resulting in an open circuit voltage (V_(oc)). Upon connection ofelectrodes, a photocurrent (short circuit current, I_(sc)) is created.

These polymeric OPV holds promise for potential cost-effectivephotovoltaics since it is solution processable. Large area OPVs havebeen demonstrated using printing (Krebs and Norrman, Using light-inducedthermocleavage in a roll-to-roll process for polymer solar cells, ACSAppl. Mater. Interfaces 2 (2010) 877-887; Krebs, et al., A roll-to-rollprocess to flexible polymer solar cells: model studies, manufacture andoperational stability studies, J. Mater. Chem. 19 (2009) 5442-5451;Krebs, et al., Large area plastic solar cell modules, Mater. Sci. Eng. B138 (2007) 106-111; Steim, et al., Flexible polymer Photovoltaic moduleswith incorporated organic bypass diodes to address module shadingeffects, Sol. Energy Mater. Sol. Cells 93 (2009) 1963-1967; Blankenburg,et al., Reel to reel wet coating as an efficient up-scaling techniquefor the production of bulk heterojunction polymer solar cells, Sol.Energy Mater. Sol. Cells 93 (2009) 476-483), spin-coating and laserscribing (Niggemann, et al., Organic solar cell modules for specificapplications—from energy autonomous systems to large area photovoltaics,Thin Solid Films 516 (2008) 7181-7187; Tipnis, et al., Large-areaorganic photovoltaic module—fabrication and performance, Sol. EnergyMater. Sol. Cells 93 (2009) 442-446; Lungenschmied, et al., Flexible,long-lived, large-area, organic solar cells, Sol. Energy Mater. Sol.Cells 91 (2007) 379-384), and roller painting (Jung and Jo,Annealing-free high efficiency and large area polymer solar cellsfabricated by a roller painting process, Adv. Func. Mater. 20 (2010)2355-2363). ITO, a transparent conductor, is commonly used as holecollecting electrode (anode) in OPV, and a normal geometry OPV startsfrom ITO anode, with the electron accepting electrode (cathode) usuallya low work function metal such as aluminum or calcium, being added viathermal evaporation process.

There are two different approaches in inverted geometry. One approach isITO-free wrap through by Zimmermann et. al., (Zimmermann, et al.,ITO-free wrap through organic solar cells—A module concept forcost-efficient reel-to-reel production. Sol. Energy Mater. Sol. Cells,2007, 91(5), 374) another approach is to add an electron transport layeronto ITO to make it function as cathode. Inverted geometry OPVs in whichthe device was built from modified ITO as cathode first have beenstudied both in single cells (Huang, et al., A Semi-transparent PlasticSolar Cell Fabricated by a Lamination Process. Adv. Mater. 2008, 20(3),415; Bang-Ying Yu, et al., Efficient inverted solar cells using TiO₂nanotube arrays. Nanotechnology, 2008, 19(25), 255202; Li, et al.,Efficient inverted polymer solar cells. Appl. Phys. Lett. 2006, 88,253503; Jingyu Zou, et al., Metal grid/conducting polymer hybridtransparent electrode for inverted polymer solar cells. Appl. Phys.Lett. 2010, 96, 203301; Waldauf, et al., Highly efficient invertedorganic photovoltaics using solution based titanium oxide as electronselective contact. Appl. Phys. Lett. 2006, 89(23), 233517; Zhou, et al.,Inverted and transparent polymer solar cells prepared with vacuum-freeprocessing. Sol. Eng. & Sol. Cells 2009, 93(4), 497) and solar modules(Krebs and Norrman, Using Light-Induced Thermocleavage in a Roll-to-RollProcess for Polymer Solar Cells. ACS Applied materials & interfaces,2010, 2, 877-87; Krebs, et al., A roll-to-roll process to flexiblepolymer solar cells: model studies, manufacture and operationalstability studies. J. of Mater. Chem. 2009, 19, 5442-51; Krebs, et al.,Large area plastic solar cell modules. Mater. Sci. Eng. B, 2007, 138(2),106-11).

In addition, to improve efficiency of the organic thin film solar cell,photoactive layers were developed using a low-molecular weight organicmaterial, with the layers stacked and functions separated by layer. (P.Peumans, V. Bulovic and S. R. Forrest, Appl. Phys. Lett. 76, 2650(2000)). Alternatively, the photoactive layers were stacked with a metallayer of about 0.5 to 5 nm interposed to double the open end voltage(V_(oc)). (A. Yakimov and S. R. Forrest, Appl. Phys. Lett. 80, 1667(2002)). As described above, stacking of photoactive layer is one of themost effective techniques for improving efficiency of the organic thinfilm solar cell. However, stacking photoactive layers can cause layersto melt due to solvent formation from the different layers. Stackingalso limits the transparency of the photovoltaic. Interposing a metallayer between the photoactive layers can prevent solvent from onephotoactive layer from penetrating into another photoactive layer andpreventing damage to the other photoactive layer. However, the metallayer also reduces light transmittance, affecting power conversionefficiency of the photovoltaic cell.

However, in order for solar cells to be compatible with windows, issueswith the transparency of the photovoltaic must first be addressed. Themetal contacts used in traditional solar modules are visibility-blockingand has to be replaced. Another challenge is to reduce cost for largescale manufacturing in order for organic solar cells to be commerciallyviable, a much lower manufacturing cost to compensate for the lowerefficiency than current photovoltaic products. OPV modules fabricated byother large scale manufacturing techniques such as printing (Krebs andNorrman, Using Light-Induced Thermocleavage in a Roll-to-Roll Processfor Polymer Solar Cells. ACS Applied materials & interfaces, 2010, 2,877-87; Krebs, et al., A roll-to-roll process to flexible polymer solarcells: model studies, manufacture and operational stability studies. J.of Mater. Chem. 2009, 19, 5442-51; Krebs, et al., Large area plasticsolar cell modules. Mater. Sci. Eng. B, 2007, 138(2), 106-11; Jung andJo, Annealing-free high efficiency and large area polymer solar cellsfabricated by a roller painting process, Adv. Func. Mater. 20 (2010)2355-2363) and spin-coating (Tipnis, et al., Large-area organicphotovoltaic module—Fabrication and performance. Sol. Energy Mater. Sol.Cells, 2009, 93(8), 442-6; Lungenschmied, et al., Flexible, long-lived,large-area, organic solar cells. Sol. Energy Mater. Sol. Cells, 2007,9(5), 379-84) have been demonstrated, however, all these still involvethe use of metal in certain way. For example, a solution-based all-spraydevice, which was opaque, showed a PCE as high as 0.42% (Lim, et al.,Spray-deposited poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)top electrode for organic solar cells, Appl. Phys. Lett. 93 (2008)193301-4). Large-scale manufacturing techniques, such as printing, havelowered the cost of manufacture, but still involve the use of metal incertain way, and therefore affect the transparency of the photovoltaiccell.

Therefore, what is needed is a new method of manufacturing organicphotovoltaic cells without the use of metal, to allow for novelphotovoltaic cells with enhanced transparency. The art at the time thepresent invention was made did not describe how to attain these goals,of manufacturing less expensive and less complex devices, havingenhanced transparency.

SUMMARY OF THE INVENTION

Comparing with conventional technology based on spin-coating and usingmetal as cathode contact, which greatly limits transparency of solarcells and posts difficulty for large scale manufacturing, the new spraytechnology solves these two problems simultaneously. A thin film organicsolar array is fabricated employing this layer-by-layer spray techniqueonto desired substrates (can be rigid as well as flexible). Thistechnology eliminates the need for high-vacuum, high temperature, lowrate and high-cost manufacturing associated with current silicon andin-organic thin film photovoltaic products.

The organic solar photovoltaic cell is manufactured on an ITO-coatedsubstrate, such as cloth, glass, plastic or any material known in theart for use as a photovoltaic substrate. Exemplary plastics include anypolymer such as acrylonitrile butadiene styrene (ABS), acrylic (PMMA),cyclic olefin copolymer (COC), ethylene-vinyl acetate (EVA), ethylenevinyl alcohol (EVOH), fluoroplastics, such as PTFE, FEP, PFA, CTFE,ECTFE, and ETFE, Kydex (an acrylic/PVC alloy), liquid crystal polymer(LCP), polyoxymethylene (POM or Acetal), polyacrylates (acrylic),polyacrylonitrile (PAN or acrylonitrile), polyamide (PA or nylon),polyamide-imide (PAI), polyaryletherketone (PAEK or ketone),polybutadiene (PBD), polybutylene (PB), polychlorotrifluoroethylene(PCTFE), polycyclohexylene dimethylene terephthalate (PCT),polycarbonate (PC), polyhydroxyalkanoates (PHAs), polyketone (PK),polyester, polyetherketoneketone (PEKK), polyetherimide (PEI),polyethersulfone (PES), chlorinated polyethylene (CPE), polyimide (PI),polymethylpentene (PMP), polyphenylene oxide (PPO), polyphenylenesulfide (PPS), polypropylene (PP), polystyrene (PS), polysulfone (PSU),polytrimethylene terephthalate (PTT), polyurethane (PU), polyvinylacetate (PVA), styrene-acrylonitrile (SAN). The ITO layer was optionallypatterned onto the first face of the glass, forming an anode, byobtaining an ITO-coated substrate, patterning the ITO usingphotolithography, etching the ITO, and cleaning the etched ITO andsubstrate. The ITO may be etched with a mixed solution of HCl and HNO₃.The etched ITO and substrate was then optionally cleaned by at least oneof acetone, isopropanol, or UV-ozone. The cleaning step may be performedat 50° C. for 20 min each, followed by drying with N₂.

A layer of Cs₂CO₃ was prepared and sprayed onto the etched ITOsubstrate. In some variations, the layer of Cs₂CO₃ was prepared bydissolving Cs₂CO₃ in 2-ethoxyethanol at a ratio of 2 mg/ml, and stirredfor 1 hour. After the Cs₂CO₃ layer was sprayed onto the OPV cell, thelayer was annealed to the OPV cell inside a glovebox. Optionally, theannealing step occurred at 150° C. for 10 min inside the N₂ glovebox.The Cs₂CO₃ layer has an optional thickness of about 5 {acute over (Å)}to about 15 {acute over (Å)}.

After the Cs₂CO₃ layer was annealed, an active layer of P3HT and PCBMwas prepared and sprayed onto the OPV cell. The active layer solutionwas optionally prepared my mixing P3HT and PCBM with a weight ratio of1:1 in dichlorobenzene. The active layer was then optionally stirred ona hotplate for 48 h at 60° C. prior to spraying. After spraying, the OPVcell was dried in an antechamber under vacuum for at least 12 hours. Theactive layer of has an optional layer thickness of about 100 nm to about500 nm, depending on the organic photovoltaic cell materials andtransparency requirements. A layer comprisingpoly(3,4)ethylenedioxythiophene:poly-styrenesulfonate and 5 vol. % ofdimethylsulfoxide was then disposed on the active layer, providing thecathode for the photovoltaic cell. Optionally, thepoly(3,4)ethylenedioxythiophene:poly-styrenesulfonate mixed with 5 vol.% of dimethylsulfoxide was prepared diluting thepoly(3,4)ethylenedioxythiophene:poly-styrenesulfonate filtering thediluted poly(3,4)ethylenedioxythiophene:poly-styrenesulfonate through a0.45 μm filter, and mixing the dimethylsulfoxide into the dilutedpoly(3,4)ethylenedioxythiophene:poly-styrenesulfonate. In somevariations, this cathodic layer has a thickness of about 100 nm to about700 nm, and may be 600 nm in some variations. Exemplary thicknessesinclude 200 nm, 250 nm, 300 nm, 350 nm, 400 nm, 450 nm, 550 nm, 600 nm,650 nm, and 700 nm.

The OPV cell was placed into high vacuum for 1 h, such as at 10⁻⁶ Torr.The OPV cell was then annealed at 120° C., 160° C., or at 120° C. for 10minutes followed by high vacuum for 1 hour and annealing at 160° C. for10 minutes and encapsulated with a UV-cured epoxy.

The photovoltaic cells may also be in electrical connection, therebyforming an array. For example, a series of organic solar photovoltaiccells disposed into an array of 50 individual cells having active areaof 12 mm². The array comprises 10 cells disposed in series in one row,and 5 rows in parallel connection in some variations.

The inventive device and method has solved the costly and complicatedprocess currently used to make crystalline and thin film solar cells,namely, high-vacuum, high temperature, low rate and high-costmanufacturing. Furthermore, this technology could be used on any type ofsubstrate including cloth and plastic. This new technology enables allsolution processable organic solar panel on with transparent contacts.This technique has great potential in large-scale, low-costmanufacturing of commercial photovoltaic products based on solutions oforganic semiconductors. The use of self assembled molecules (SAM) modifythe work function of ITO, and SAM was used in place of the previousCs₂CO₃ improving the device efficiency and reproducibility.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the invention, reference should be made tothe following detailed description, taken in connection with theaccompanying drawings, in which:

FIG. 1 is a diagram showing a perspective view of the novel inverted OPVcells containing sprayed-on layers.

FIGS. 2(A) and (B) are images of the device structure of an invertedtest device. (A) top view; (B) side view.

FIG. 3 is a graph showing the I-V characteristics of three test deviceswithout Cs₂CO₃ layer (black solid line), and with Cs₂CO₃ layer atdifference thickness (black line with empty triangle and line withfilled triangle).

FIGS. 4(A) and (B) are graphs showing the comparison of (A) transparencyand (B) resistance between ITO and the anode (modified PEDOT:PSS) atdifferent thickness.

FIG. 5 is a graph showing the transmission spectra of an active layer(P3HT:PCBM) of 500 nm (black line with filled square), and with am-PEDOT:PSS layer of 600 nm (gray line with filled circle).

FIG. 6 is a top-view image of the device architecture of an invertedarray having 50 cells in the array.

FIG. 7 is a side-view image of the device architecture of an invertedarray.

FIG. 8 is a graph showing the IV of four test cells measured with AM1.5solar illumination under various annealing conditions: 1-step annealingat 120° C. (light grey filled circle), or 160° C. (black filled square),and 2-step annealing (dark grey filled triangle).

FIG. 9 is a graph showing the IPCE of four test cells measured undertungsten lamp illumination at various annealing conditions: 1-stepannealing at 120° C. (light grey filled circle), or 160° C. (blackfilled square), and 2-step annealing (dark grey filled triangle).

FIG. 10 is a graph showing the IV of 4 inverted spray-on array measuredwith AM1.5 solar illumination under various annealing conditions: 1-stepannealing at 120° C. (dashed line), or 160° C. (light grey thin line),and 2-step annealing (black filled square). These 3 arrays use m-PEDOT500 as anode. The 4^(th) array (thick dark grey line) uses m-PEDOT 500as anode and was annealed at 160° C.

FIG. 11 is a graph showing the improvement of IV of an inverted arrayunder continuous AM1.5 solar illumination. The first measurement wasdone right after the array was fabricated and encapsulated.

FIG. 12 is an image showing the transparency of manufactured sprayedsolar array using the disclosed methods.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention for the fabrication of a see-through organic solararray via layer-by-layer (LBL) spray may be understood more readily byreference to the following detailed description of the preferredembodiments of the invention and the Examples included herein. However,before the present compounds, compositions, and methods are disclosedand described, it is to be understood that this invention is not limitedto specific compounds, specific conditions, or specific methods, etc.,unless stated as such. Thus, the invention may vary, and the numerousmodifications and variations therein will be apparent to those skilledin the art. It is also to be understood that the terminology used hereinis for the purpose of describing specific embodiments only and is notintended to be limiting.

As used herein, “about” means approximately or nearly and in the contextof a numerical value or range set forth means±15% of the numerical.

As used herein, “substantially” means largely if not wholly that whichis specified but so close that the difference is insignificant.

All masks for spray are custom made by Towne Technologies, Inc. Theairbrush sets for spray was purchased from ACE hardware.

Example 1

The indium tin oxide (ITO) was patterned onto a Corning® low alkalineearth boro-aluminosilicate glass having a nominal sheet resistance of4-10 Ohm/square (Delta Technology, Inc.) using standard photolithographymethod and cleaned following the procedure described elsewhere (Lewis,et al., Fabrication of organic solar array for applications inmicroelectromechanical systems. Journal of Renewable and SustainableEnergy 2009, 1, 013101-9). The substrate is then exposed to a UV-lampfor 1.4 seconds in a constant intensity mode set to 25 watts. Thestructure was developed for about 2.5 minutes using Shipley MF319 andrinsed with water. The substrate was then hard-baked, at 145° C. for 4minutes and any excess photoresist cleaned off with acetone and cotton.After cleaning, the substrate was etched from about 5-11 minutes with asolution of 20% HCl-7% HNO₃ on a hotplate at 100° C. The etchedsubstrate was then cleaned by hand using acetone followed by isopropanoland UV-ozone cleaned for at least 15 minutes.

An interstitial layer was formed on top of the patterned ITO layer. Asolution of 0.2% wt. Cs₂CO₃ (2 mg/mL; Sigma-Aldrich Co. LLC, St. Louis,Mo.) in 2-ethoxyethanol was prepared and stirred for one hour at roomtemperature. Cs₂CO₃ was chosen to reduce ITO work function close to 4.0eV to be utilized as cathode. The Cs₂CO₃ solution was sprayed onto theclean ITO substrate through a custom made shadow mask with an airbrushusing N₂ set to 20 psi from a distance of about 7-10 centimeters. Theproduct was then annealed for 10 minutes at 150° C. in an N₂ glovebox(MOD-01; M. Braun Inertgas-Systeme GmbH, Garching German).

The active layer solution was prepared by mixing separate solutions of ahigh molecular weight poly(3-hexylthiophene (P3HT with regioregularityover 99% and average molecular weight of 42K; Rieke Metals, Inc.,Lincoln, Nebr.) and 6,6-phenyl C61 butyric acid methyl ester (PCBM, C₆₀with 99.5% purity; Nano-C, Inc., Westwood, Mass.) at a weight ratio of1:1 in dichlorobenzene at 20 mg/mL and stirred on a hotplate for 48hours at 60° C. The active coating was then spray coated onto the Cs₂CO₃coated substrate using an airbrush with N₂ set to 30 psi. The airbrushwas set at about 7-10 cm away from the substrate and multiple lightlayers of active layer were sprayed, resulting in a layer thickness ofabout 200 to about 300 nm. The device is then left to dry in theantechamber under vacuum for at least 12 hours. After drying, excessactive layer solution was wiped off of the substrate usingdichlorobenzene (DCB)-wetted cotton followed by isopropanol-wettedcotton.

A kovar shadow mask was aligned in position with the substrate and heldin place by placing a magnet underneath the substrate. The seriesconnection locations were wiped using a wooden dowel to expose thecathode for later electrical connection. The original aqueouspoly(3,4)ethylenedioxythiophene:poly-styrenesulfonate (PEDOT:PSS,Baytron 500 and 750; H. C. Starck GmbH, Goslar Germany) was diluted andfiltered out through a 0.45 μm filter. This filtered solution ofPEDOT:PSS is mixed with 5 vol. % of dimethylsulfoxide to increaseconductivity (Lim, et al., Spray-depositedpoly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) top electrodefor organic solar cells. Appl. Phys. Lett. 2008, 93, 193301). Thesolution was then stirred at room temperature followed by 1 h ofsonification. The m-PED coating was prepared by placing a substrate/maskon a hotplate (90° C.). The m-PED layer was spray coated using nitrogen(N₂) as the carrier gas, set to 30 psi, with the airbruch positionedabout 7-10 cm from the substrate. Multiple light layers were applieduntil the final thickness of about 500 nm to about 700 nm was reached.The substrate was then removed from the hotplate and the mask removed.Care was taken to avoid removing the mPED with the mask. The substratewas placed into high vacuum treatment (10⁻⁶ Torr) for 1 h, followed by asubstrate annealing at 120-160° C. for 10 min. The modified PEDOT:PSS(m-PEDOT) was then sprayed onto the substrate using a custom made spraymask.

The finished device was placed into high vacuum (10⁻⁶ Torr) for 1 h.This step was shown to improve the device performance with sprayedactive layer (Lim, et al., Spray-depositedpoly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) top electrodefor organic solar cells. Appl. Phys. Lett. 2008, 93, 193301). The finaldevice was annealed at various conditions, including 120° C., 160° C.,and step annealing comprising 120° C. for 10 minutes followed by highvacuum for 1 hour and annealing at 160° C. for 10 minutes. The annealeddevice was encapsulated using a UV-cured encapsulant (EPO-TEK OG142-12;Epoxy Technology, Inc., Billerica, Mass.) was applied to the edge of theencapsulation glass, and the glass is placed into the glovebox for atleast 15 min, with UV exposure. The device was then flipped upside down,and the epoxy applied on top of the encapsulation glass. The device wasfinally exposed to 15 min of UV to cure the encapsulant epoxy.

Example 2

Inverted organic photovoltaic cell 1, seen in FIG. 1, was created usingthe method described in Example 1, using pre-cut 4″×4″ ITO glasssubstrates with a nominal sheet resistance of 4-10 Ohm/square andCorning® low alkaline earth boro-aluminosilicate glass (DeltaTechnology, Inc., Tallahassee, Fla.). Inverted photovoltaic cell 1 wascomposed of different layers of active materials and terminals (anodeand cathode) built onto substrate 5. Anode 10, comprised of ITO in thepresent example, was sprayed onto substrate 5 forming a ‘

’ pattern extending from a first set of edges of substrate 5.Interstitial layer 40 covers anode 10, except for the outermost edges,as seen in FIG. 2(A), and permits ITO to be used as an anode asdiscussed in Example 1. The components of the SAM layer were chosen toprovide a gradient for charges crossing the interface, approximating aconventional p-n junction with organic semiconductors, thereby providingan increased efficiency of heterojunctions. Active layer 30 is disposeddirectly on top of interfacial buffer layer 40, and was prepared usingpoly(3-hexylthiophene) and 6,6-phenyl C61 butyric acid methyl ester.Anode 20 was disposed on the active layer in a pattern, similar to thecathode, but perpendicular to the cathode. Exemplary anode materialsinclude PEDOT:PSS doped with dimethylsulfoxide. The fully encapsulated 4μm×4 μm array was found to possess over 30% transparency.

Example 3

An inverted single-cell test device was used as a starting point toensure a good reference point for the multi-cell array, which consistsof four identical small cells (4 mm²) on a 1″×1″ substrate, as seen inFIG. 2(B). The cell is sandwiched between two cross electrodes,designated as 50 and 51. The test device was fabricated using the sameprocedure described in Example 1, with m-PEDOT 500 as anode.

ITO normally has a work function of ˜4.9 eV. The function of ITO in atraditional OPV device is as an anode. There have been previous reportson tuning the work function of ITO by adding an electron transport layersuch as ZnO (Jingyu Zou, et al., Metal grid/conducting polymer hybridtransparent electrode for inverted polymer solar cells. Appl. Phys.Lett. 2010, 96, 203301), TiO₂ (Huang, et al., A Semi-transparent PlasticSolar Cell Fabricated by a Lamination Process. Adv. Mater. 2008, 20(3),415; Bang-Ying Yu, et al., Efficient inverted solar cells using TiO₂nanotube arrays. Nanotechnology, 2008, 19(25), 255202; Li, et al.,Efficient inverted polymer solar cells. Appl. Phys. Lett. 2006, 88,253503), PEO (Zhou, et al., Inverted and transparent polymer solar cellsprepared with vacuum-free processing. Sol. Eng. & Sol. Cells 2009,93(4), 497) and Cs₂CO₃ (Huang, et al., A Semi-transparent Plastic SolarCell Fabricated by a Lamination Process. Adv. Mater. 2008, 20(3), 415;Bang-Ying Yu, et al., Efficient inverted solar cells using TiO₂ nanotubearrays. Nanotechnology, 2008, 19(25), 255202; Li, et al., Efficientinverted polymer solar cells. Appl. Phys. Lett. 2006, 88, 253503) ininverted OPV single cells. In this work, Cs₂CO₃ was chosen for itseconomic cost and easy handling. By spin coating a solution of2-ethoxyethanol with 0.2% Cs₂CO₃ at 5000 rpm for 60s, a very thin layer(˜10 {acute over (Å)}) of Cs₂CO₃ is formed over the ITO. It was reportedthat a dipole layer would be created between Cs₂CO₃ and the ITO. Thedipole moment helped to reduce the work function of ITO, allowing ITO toserve as the cathode (Huang, et al., A Semi-transparent Plastic SolarCell Fabricated by a Lamination Process. Adv. Mater. 2008, 20(3), 415;Bang-Ying Yu, et al., Efficient inverted solar cells using TiO₂ nanotubearrays. Nanotechnology, 2008, 19(25), 255202; Li, et al., Efficientinverted polymer solar cells. Appl. Phys. Lett. 2006, 88, 253503).

FIG. 3 shows how the Cs₂CO₃ layer affects the performance of theinverted cell. The control cell without Cs₂CO₃ (black solid line)performed almost like a resistor and had negligible V_(oc) (0.03V).Without being bound to any specific theory, the difference between thepresent invention and previous work (Zhou, et al., Inverted andtransparent polymer solar cells prepared with vacuum-free processing.Sol. Eng. & Sol. Cells 2009, 93(4), 497) can be explained by the use ofan electron transport layer to alleviate non-ohmic contact with thecathode (PEDOT in this case) in their work. For a better controlledthickness, Cs₂CO₃ was spin coated on to the cleaned ITO substrate inthese devices. As shown in FIG. 3, the optimal thickness of Cs₂CO₃ layerwas achieved at a spin rate of 5000 rpm. At higher rate of 7000 rpm, thedevice was less efficient owing to the fact of a discontinuous Cs₂CO₃layer. The optimal thickness was later determined to be around 15 Å.

Previous reports showed Cs₂CO₃ can lower the ITO work function to as lowas 3.3 eV (Huang, et al., A Semi-transparent Plastic Solar CellFabricated by a Lamination Process. Adv. Mater. 2008, 20(3), 415;Bang-Ying Yu, et al., Efficient inverted solar cells using TiO₂ nanotubearrays. Nanotechnology, 2008, 19(25), 255202; Li, et al., Efficientinverted polymer solar cells. Appl. Phys. Lett. 2006, 88, 253503). Inorder to get an estimate of the effective work function of ITO/Cs₂CO₃, acontrol device with aluminum (100 nm in thickness) as cathode wasfabricated. Since aluminum is not transparent, the I-V was measured byshining light from m-PEDOT side. The device was analyzed by exposing thecell to continuous radiation. The current-voltage (I-V) characterizationof the solar array was performed with a 1.6 KW solar simulator underAM1.5 irradiance of 100 mW/cm² (Newport Corp., Franklin Mass.). Nospectral mismatch with the standard solar spectrum was corrected in thepower conversion efficiency (PCE) calculation. The incident photonconverted electron (IPCE), or the external quantum efficiency, of thedevice was measured using 250 W tungsten halogen lamp coupled with amonochromator (Newport Oriel Cornerstone 1/4 m). The photocurrent wasdetected by a UV enhanced silicon detector connected with a Keithley2000 multimeter. The transmission spectrum of active layer was performedon the same optical setup. V_(oc) of such control device was 0.24V,whereas V_(oc) of the inverted cell in FIG. 3 was 0.36V measured underthe same illumination condition. Since aluminum has work function of 4.2eV, this means in the present invention, the effective work function ofITO/Cs₂CO₃ is close to 4.1 eV.

Example 4

An inverted single-cell test device was prepared, as discussed inExample 1, but using different thicknesses of m-PEDOT to determine cellcharacteristics at different cell thicknesses. ITO was chose as areference for comparison. At thickness of about 100 nm, the transparencyof m-PEDOT is about 80%, comparable with ITO, as seen in FIG. 4(A). Asexpected, the resistance decreases as thickness increases, which isconsistent with the bulk model, seen in FIG. 4(B). The trade-off betweentransparency and resistance is another important fabrication parameter.The current array was fabricated with thickness of about 600 nm, whichhas moderate resistance of 70 ohm/square, and transparency about 50%. Acomparison between transmission spectra of the active layer (P3HT:PCBM,200 nm) and m-PEDOT anode of 600 nm showed the total transparency overthe spectra range shown decreases from 73% to 31% after spraying on them-PEDOT anode, as seen in FIG. 5.

A solar array was prepared, as disclosed above, comprising 50 individualcells each has active area of 12 mm², seen in FIG. 6. The array wasconfigured with 10 cells in series to increase in one row to increasevoltage, and 5 rows in parallel connection to increase current, seen incross section in FIG. 7. The arrays were prepared with have m-PEDOT 750or m-PEDOT 500 as semitransparent anode.

Example 5

Annealing has shown to be the most important factor to improve organicsolar cell performance (Shaheen, Brabec, Sariciftci, Padinger, Fromherz,and Hummelen, Appl. Phys. Lett. 2001, 78, 841; Padinger, et al., Effectsof Postproduction Treatment on Plastic Solar Cells. Adv. Funct. Mater.2003, 13(1), 85-88). Cells were exposed to a 1.6 KW solar simulatorunder AM1.5 irradiance of 100 mW/cm² (Newport Corp., Franklin Mass.).Current-voltage (IV) and incident photon converted electron (IPCE) werecompared between three inverted test cells at different annealingconditions, as seen in FIG. 8: 1-step annealing at 120° C. (gray filledcircle), or 160° C. (black filled square); 2-step annealing at 120° C.for 10 minutes, followed by high vacuum for 1 hour and annealing at 160°C. for 10 minutes. One-step annealing at 120° C. gives the best resultin test cell, as seen in FIG. 8, with V_(oc)=0.48V, I_(sc)=0.23 mA,FF=0.44, and a power conversion efficiency (PCE) of 1.2% under AM1.5solar illumination with intensity 100 mW/cm². The second annealing stepat 160° C. worsens the device performance, mainly due to unfavorablechange of film morphology, which was confirmed in AFM images (data notshown). The PCE of 1-step annealing at 160° C. was in between that of1-step annealing at 120° C. and 2-step annealing, yet the device has theworst FF. Table 1 listed the details of the IV characteristics of thesethree test cells.

TABLE 1 Test cell I-V characteristics comparison at various annealingconditions. Test cell Annealing number I_(sc) (mA) V_(oc) (V) FF η (%)condition 1 0.28 0.48 0.26 0.86 160° C. 10 min 2 0.23 0.48 0.44 1.2 120°C. 10 min 3 0.16 0.30 0.35 0.43 2-step

IPCE measurement shows 2-step annealing was worse than 1 step annealing,seen in FIG. 9, which was consistent with IV measurements (data notshown). There is some inconsistency between PCE and IPCE for the cellsannealed at 160° C. and 120° C.: the cell annealed at 160° C. has higherIPCE yet lower PCE than that at 120° C. IPCE measurement was done underillumination from Tungsten lamp, whereas IV was done under solarsimulator which has different spectrum than that of the tungsten lamp.Nevertheless, the integration of IPCE should be proportional to I_(sc).The device made by 1-step annealing at 160° C., though having smallerpower conversion efficiency, actually has larger I_(sc) (0.28 mA) thanthe one at 120° C. (0.23 mA). The ratio between integral of IPCE at 160°C. vs. 120° C. is about 1.3, and the ratio of I_(sc) of the same deviceswas 1.2. The slight discrepancy might also come from the fact that thecells behave differently under strong (IV) and weak (IPCE)illuminations. Usually bi-molecular (BM) recombination sets in underhigh light intensity (solar simulator) (Shaheen, Brabec, Sariciftci,Padinger, Fromherz, and Hummelen, Appl. Phys. Lett. 2001, 78, 841)meaning the cell which has more prominent BM recombination will performpoorer with high intensity illumination such as that from the solarsimulator. It might be that the cell annealed at 160° C. was affected byBM recombination more than the cell annealed at 120° C., due to moretraps associated with rougher morphology serving as recombinationcenters. Further investigation of this discrepancy is under study.

AFM images of topography and phase of 4 different test arrays atdifferent annealing conditions; an as-made cell, made using the methodof Example 1 without annealing, having a roughness of 7.41 nm, 1-stepannealing at 120° C. having a roughness of 6.60 nm, annealing at 160° C.having a roughness of 3.68 nm, and (d) 2-step annealing having aroughness of 9.76 nm. The 1-step annealing at 120° C. showed theimproved film roughness and the best phase segregation of P3HT and PCBM,which explains why the device performance was the best, seen in FIGS. 8and 9. Device by 2-step annealing has the smoothest film, however, thephase segregation was much less distinct. This indicates that P3HTchains and PCBM molecules are penetrating through each other more afterthe second annealing at 160° C., and form much smaller nano-domains,which are favorable for charge transport between the domains (Kline andMcGehee, Morphology and Charge Transport in Conjugated Polymers. J ofMacromol Sci, Part C: Polymer Reviews, 2006, 46(1): 27-45). However,recombination of photogenerated carriers might be enhanced due to thelack of separate pathways for electron sand holes, and that was why thedevice after 2-step annealing performed worse than after the 1^(st)annealing at 120° C., seen in FIGS. 8 and 9. 1-step annealing at highertemperature of 160° C. results in the roughest film (even rougher thanthe as-made device), and the P3HT phase and PCBM phase are hardlydistinguishable. This rough film also further affects the interfacebetween active layer and m-PEDOT, resulting in poor FF of the device,seen in FIGS. 8 and 9.

IV analysis was performed on 4 arrays under different annealingconditions measured with AM1.5 solar illumination, seen in FIG. 10. Itis clear that 1-step annealing at the low temperature, i.e. 120° C.,gives the worst result, 2-step annealing showed improved IVcharacteristics (V_(oc), J_(sc), FF and PCE) after the second hightemperature annealing at 160° C. 1-step annealing at high temperature,i.e. 160° C., gives the best V_(oc), and 2-step annealing yields thehighest J_(sc). In terms of anode, m-PEDOT 500 seems to give higherV_(oc) than PEDOT 750, seen in Table 2. However, there is not muchdifference in PCE between 2-step annealing and 1-step annealing at 160°C., which is in contrast with the result of test device, seen in FIGS. 8and 9. It is believed the annealing duration is probably too short forthe array, since it has much larger area and contains much morematerials.

TABLE 2 Array I-V characteristics comparison at various annealingconditions. Array I_(sc) V_(oc) η Annealing number (mA) (V) FF (%)condition m-PEDOT 1 17.0 3.9 0.30 0.68 2 step 750 2 11.5 4.0 0.39 0.62 2step 750 3 6.30 2.8 0.37 0.22 2 step 750 4 13.0 4.0 0.33 0.56 160° C. 10min 750 5 15.0 5.2 0.33 0.86 160° C. 10 min 500 6 12.0 5.8 0.30 0.70160° C. 10 min 500 7 11.1 5.2 0.35 0.67 160° C. 10 min 500

A very interesting phenomenon which was termed ‘photo annealing’ wasobserved, as seen in FIG. 11. Under constant illumination from the solarsimulator, a sudden change of IV occurs after certain amount of timewhich is device dependent, ranging from 10 minutes to several hours. Thedevice takes about 15 minutes, and reaches maximum PCE after 2.5 hoursunder illumination. The drastic change is mostly I_(sc), which more thandoubles from 17 mA to 35 mA after 2.5 hours. The change of V_(oc) wasmarginal from 4.0V to 4.2V. The maximum PCE of the array was 1.80%.Table 3 listed the changes of other IV characteristics.

TABLE 3 Change of Array IV characteristics under solar illumination.Time I_(sc) (mA) V_(oc) (V) FF η (%) 1^(st) day  0 min 17 4.0 0.30 0.68 12 min 28 4.2 0.35 1.40 150 min 35 4.2 0.37 1.80 2^(nd) day  0 min 184.2 0.35 0.88

Furthermore, this sudden increase of I_(sc) is also accompanied by acharacteristic ‘wiggles’ on the IV curve. This cannot be due toencapsulation related change of light distribution inside the activelayer, since these ‘wiggles’ have also been observed with the IV of testdevices which are not encapsulated. ‘Wiggles’ only appear with thesprayed OPV device, both array and test device, not with spin-coateddevice. Without being bound to any specific theory, the phenomenon maybe a result of the porosity of sprayed film being much larger than thespin-coated film, and polymer chains have much more loose arrangement insprayed device, with the heat from solar illumination, the polymerchains relax more and the film nanomorphology was improved, withpossibly PCBM penetrating into the voids between polymer chains andcausing better phase segregation (Geiser, et al.,Poly(3-hexylthiophene)/C₆₀ heterojunction solar cells: Implication ofmorphology on performance and ambipolar charge collection. Sol. Eng. &Sol. Cells 2008, 92(4), 464). This effect is similar to thermalannealing performed on hot plate. As temperature drops down, the polymerchains go back to its original configuration, and IV curve is back toits original one, manifesting certain kind of hysteresis. It also mightbe due to thermal activation of the previous deeply trapped carriers(i.e., polarons), which results in increased photocurrent at highertemperature (Graupner, Leditzky, Leising, and Scherf, Phys. Rev. B 1996,54, 7610; Nelson, Organic photovoltaic films. Current Opinion in SolidState and Materials Science 2002, 6(1), 87-95). The wiggles indicate thenonuniformity of film morphology, and the overall boost of deviceperformance is the result of ‘photo annealing’.

This observation is against the conventional picture of organic solarcell, which normally shows degradation under solar illumination (Nelson,Organic photovoltaic films. Current Opinion in Solid State and MaterialsScience 2002, 6(1), 87-95; Dennler, et al., A new encapsulation solutionfor flexible organic solar cells. Thin Solid Films 2006, 511-512,349-53). It was also found out that the performance enhancement underillumination only happened with sprayed devices, not the device made byspin coating. This means that solar cells made with our spray-ontechnique performs better under sunlight, which is beneficial for solarenergy application. Further study of photo annealing dynamics and solararray lifetime is ongoing to unveil the optimal condition for solararray in field operations.

Example 6

A large area organic array was fabricated using the all spay techniquedescribed in Example 1. A fully encapsulated 4″×4″ array was preparedand found to have over 30% transparency, with power conversionefficiency (PCE) as high as 1.80% under constant illumination ofsimulated sunlight. Thermal annealing has proven to be essential toimprove device PCE, and the optimal annealing conditions are not thesame with small single cell and large solar array consisting of 50cells. Systematic studies of optical, electronic and morphologicproperties of the device reveals the influence of nanomorphology overdevice power conversion efficiency. Moreover, the discovery of photoannealing, i.e., more than 2-fold increase of solar cell PCE under solarirradiance and with hysteresis pattern, is in contrary to the normalunderstanding of organic solar cell degradation under sunlight. The factthat photo annealing was only observed with sprayed solar cell or arraysplaces underscores the novel advantageous solution for large scale,low-cost solution based solar energy applications. Analysis of thedevice showed that the solar array provided useful device transparency,as seen in FIG. 12.

In the preceding specification, all documents, acts, or informationdisclosed do not constitute an admission that the document, act, orinformation of any combination thereof was publicly available, known tothe public, part of the general knowledge in the art, or was known to berelevant to solve any problem at the time of priority.

The disclosures of all publications cited above are expresslyincorporated herein by reference, each in its entirety, to the sameextent as if each were incorporated by reference individually.

While there has been described and illustrated specific embodiments ofan organic photovoltaic cell, it will be apparent to those skilled inthe art that variations and modifications are possible without deviatingfrom the broad spirit and principle of the present invention. It isintended that all matters contained in the foregoing description orshown in the accompanying drawings shall be interpreted as illustrativeand not in a limiting sense. It is also to be understood that thefollowing claims are intended to cover all of the generic and specificfeatures of the invention herein described, and all statements of thescope of the invention which, as a matter of language, might be said tofall therebetween.

What is claimed is:
 1. An organic solar photovoltaic cell, manufacturedcomprising the steps: obtaining an indium tin oxide substrate; sprayinga layer of Cs₂CO₃ onto the etched indium tin oxide substrate; annealingthe a layer of Cs₂CO₃ inside a glovebox; spraying an active layer ofpoly(3-hexylthiophene and 6,6-phenyl C61 butyric acid methyl ester onthe layer of Cs₂CO₃; wherein the active layer of poly(3-hexylthiopheneand 6,6-phenyl C61 butyric acid methyl ester has a final layer thicknessof about 200 nm to about 300 nm; drying the device in an antechamberunder vacuum for at least 12 hours; spraying a layer comprisingpoly(3,4)ethylenedioxythiophene:poly-styrenesulfonate mixed with 5 vol.% of dimethylsulfoxide on the active layer; placing the organic solarphotovoltaic cell into high vacuum for 1 h; annealing the solarphotovoltaic cell, wherein the annealing is performed at 120° C., theannealing is performed at 160° C., or the annealing is performed at 120°C. for 10 minutes followed by high vacuum for 1 hour and annealing at160° C. for 10 minutes; and encapsulating the organic solar photovoltaiccell with a UV-cured epoxy.
 2. The organic solar photovoltaic cell ofclaim 1, wherein the substrate is low alkaline earthboro-aluminosilicate glass, cloth, or plastic.
 3. The organic solarphotovoltaic cell of claim 2, wherein the cloth is nylon cloth, cottoncloth, polyester cloth, hemp cloth, bamboo cloth.
 4. The organic solarphotovoltaic cell of claim 1, wherein the active layer of has a layerthickness of about 500 nm.
 5. The organic solar photovoltaic cell ofclaim 1, further comprising a series of organic solar photovoltaic cellsdisposed into an array of 50 individual cells having active area of 12mm².
 6. The organic solar photovoltaic cell of claim 5, wherein thearray further comprises 10 cells disposed in series in one row, and 5rows in parallel connection.
 7. The organic solar photovoltaic cell ofclaim 1, further comprising four identical cells disposed on a 1″×1″substrate.
 8. The organic solar photovoltaic cell of claim 1, whereinthe Cs₂CO₃ layer is about 5 {acute over (Å)} to about 15 {acute over(Å)} thick.
 9. The organic solar photovoltaic cell of claim 1, whereinthe Cs₂CO₃ layer is about 15 {acute over (Å)} thick.
 10. The organicsolar photovoltaic cell of claim 1, wherein the thickness of the layercomprising poly(3,4)ethylenedioxythiophene:poly-styrenesulfonate and 5vol. % of dimethylsulfoxide is about 100 nm to about 600 nm.
 11. Theorganic solar photovoltaic cell of claim 10, wherein the thickness ofthe active layer is about 100 to about 300 nm, or about 200 nm.
 12. Theorganic solar photovoltaic cell of claim 10, wherein the thickness ofthe active layer is 200 nm and the thickness of the layer comprisingpoly(3,4)ethylenedioxythiophene:poly-styrenesulfonate and 5 vol. % ofdimethylsulfoxide is about 100 nm-600 nm, about 100 nm, or 600 nm.